A filter which digitally processes input signals at a realtime operation has been discussed in, for instance, "PERFORMANCE EVALUATIONS OF SELECTED AUTOMATIC DEGHOSTING SYSTEMS FOR TELEVISION" in "IEEE Transactions on Consumer Electronics, Vol. CE-26, February 1980".
FIG. 1 shows a conventional six tap input-weighted transversal filter (hereinafter referred generally to as a transversal filter). This transversal filter shows a part of a large-scale transversal filter provided with many taps.
An input signal a={x(i)}, sampled at every period T, is inputted to six tap weight multipliers 10, 11, 12, 13, 14, 15 via an input terminal 1. The tap weight multipliers 10, 11, 12, 13, 14, 15 multiply the input signal a by respective tap weights C0, C1, C2, C3, C4, C5 associated thereto and output their multiplication results to adders 20, 21, 22, 23, 24, 25, respectively. Further, a cascade input signal e={z(i)} is input to the adder 25 through a cascade inputted terminal 3. The adders 20, 21, 22, 23, 24, 25 have been connected in series so that they are positioned alternately with delay elements 30, 31, 32, 33, 34 and the output from the adder 20 is fed to an output terminal 2 through a last stage delay element 4. The delay elements 30, 31, 32, 33, 34, 4 are driven by a clock signal CK.
An output signal h={y(i)} of the last stage delay element 4 of the transversal filter is expressed by the following equation: ##EQU1## c(j) represents tap weight values, where j is a number of tap weights. X(i-j-1) is an input signal which is sampled over a period of time. Z(i-6) represents a cascade input signal at a particular period of time.
This output signal {y(i)} will become the final output signal of the transversal filter.
The cascade input terminal 3 is provided for coupling these transversal filters in series. Thus, when the transversal filter is one element, such a cascade input terminal is omitted from the circuit arrangement.
The arithmetic operation shown by the equation (1) has been used in many fields. In particular, in the fields of television deghosting technique and a digital processing technique of television signals, it is used at T=70 to 93 ns (10.sup.-9 sec.) and to execute the equation (1) at high speed in a realtime operation, ROMs (Read Only Memories) and RAMs (Random Access Memories) have been used for the tap weight multipliers 10, 11, 12, 13, 14, 15 in addition to random logic multipliers.
Now, when considering a fabrication of the transversal filter onto an integrated circuit (IC) chip, the circuit-scale of these tap weight multipliers becomes relatively large and the number of taps that can be secured by one chip is no more than 64 taps. Normally, in deghosting devices transversal filters are used at 14.32 MHz, which is four times of the color subcarrier frequency fsc at the NTSC system. Thus, the period is calculated as T=70 ns. At this time, a 64 tap transversal filter will have a sampling period of 4.4 .mu.s. It is generally said that the 90% of ghosts are generated in a range of delay time from -1 to 24 .mu.s. To realize a practical deghosting device that copes with ghosts in this range, transversal filters having more than 357 taps are required. In other words, more than six IC chips become necessary and it becomes expensive for reasons that many component parts are required and required chip-scale becomes large.
As described above, conventional transversal filters enabling realtime operation at high speed have such problems that the circuit-scale of the tap weight multiplier becomes large and a sufficient number of taps cannot be obtained even when the filter is made in IC chips.